1. Field of the Invention
This invention relates generally to the field of alkyl esters. More particularly, the invention relates to a method of making alkyl esters from low-cost feedstock.
2. Background of the Invention
Alkyl esters, including methyl ester or ethyl ester, also known as “biodiesel”, are a renewable and clean burning alternative to conventional petroleum-derived diesel fuel. Biodiesel is made from a raw or used vegetable oil or animal fat, typically soybean oil or rapeseed oil. Because biodiesel is made from natural oil or fat sources, the alkyl esters typically comprise C14 to C18 fatty chains if derived from vegetable oil, and C16 to C22 fatty chains if derived from animal fat. Biodiesel can be combusted in diesel (combustion-ignition) engines either in pure form or as blended with petroleum-derived diesel fuel. Biodiesel provides the benefits of a renewable resource as well as providing lower sulfur emissions than petroleum diesel. Biodiesel is effectively a zero-sulfur emission fuel.
Alkyl esters can be produced from any vegetable oil source and can be made from the crude oil, or from oils that have been processed by filtration, refining, or other processing steps. Additionally, alkyl esters may be derived from various grades of vegetable oil including virgin oils, yellow grease (free fatty acid content of up to 15%) brown grease (flee fatty acid content of greater than 15%), or by-products of the edible oil refining process such as acidulated soap stock. Acidulated soap stock is typically made by reacting soap by-product from the vegetable oil refining process with sulfuric acid, and is composed of over 70% free fatty acid. Each grade of vegetable oil contains varying amounts of triglycerides, diglycerides, monoglycerides, free fatty acids, and glycerin, as well as other impurities. Suitable feedstocks include not only high quality refined vegetable oils, but lower grade oils such as degummed oil, once refined oil such as acidulated soybean soap stock, and animal fats such as tallow, poultry fats and used greases.
The feedstock costs of fuels contribute very significantly to the final cost of the fuel product. Hence, it is desirable to use lower-cost oil feedstocks that contain higher levels of free fatty acid. However, alkyl esters are commonly made by processes that require highly refined vegetable oils. For example, the base-catalyzed transesterification of triglycerides with an alcohol such as methanol is widely understood. In these processes, a homogeneous catalyst is used (one that dissolves into the feedstock mixture). Base catalyzed transesterification is preferred over acid catalyzed transesterification because the reaction rate for converting triglycerides to alkyl esters (and glycerin co-product) is much higher. Unfortunately, the transesterification processes cannot tolerate high levels of free fatty acid because soap by-product is produced, and complicates the product recovery and purification and irreversibly consumes a portion of the catalyst. Therefore base catalyzed processes typically require a highly refined vegetable oil feedstock that is considerably more expensive than lower grade unrefined oil feedstocks.
Consequently, there remains a need for in the art for methods of making alkyl esters using unrefined and low-cost feedstocks.